The present invention relates to a self-emission display apparatus and a method of driving the same.
The present application claims priority from Japanese Application No. 2005-202963, the disclosure of which is incorporated herein by reference.
A self-emission display apparatus equipped with self-emission elements such as organic EL elements (acting as essential elements) has been used as an information displaying device, and an illuminating light source such as a general illumination, a back light or one of other kinds of light sources, and has been expected for use as a new type of display device such as a paper display.
Each of self-emission elements acting as essential elements for a self-emission display apparatus has a basic structure formed by interposing a semiconductor layer having p-n junction between an anode (or hole injection electrode) and a cathode (or electron injection electrode). If a self-emission element is a low molecule type organic EL element, the semiconductor layer will being laminated structure comprising organic layers including a luminescent layer. On the other hand, if a self-emission element is a high molecule type organic EL element, the semiconductor layer will be comprised of organic layers providing a laminated structure formed by laminating one or more bipolar material layers. In this way, when a voltage is applied between the two electrodes which are the anode and the cathode, positive holes injected and transported from the anode into the organic layers and electrodes injected and transported from the cathode into the organic layers will be recombined with each other within an organic layer (for example, luminescent layer), thereby producing an energy from an excited state formed due to the recombination, thus effecting an emission of light.
FIG. 1A is a graph showing a current-voltage characteristic of an organic EL element. According to such a characteristic, when a driving voltage (a voltage in a forward direction) larger than an emission threshold voltage V this applied to the organic EL element, it is possible to obtain an emission brightness L proportional to an electric current corresponding to the driving voltage. On the other hand, if an applied driving voltage is equal to or lower than the emission threshold voltage Vth, there will be no driving current and an emission brightness will stay at a value equal to zero.
Thus, with regard to a self-emission element capable of providing an emission brightness proportional to an electric current, it is easy to set a desired brightness by performing a constant-current driving. However, a constant-current driving is usually associated with a complex peripheral circuit.
Accordingly, with respect to a self-emission element such as an organic EL element having a current-brightness characteristic, an often used driving method usually employs a constant-voltage driving device which is cheap and easily available. During such a constant-voltage driving, as shown in FIG. 1A, a self-emission element such as an organic EL element will change in its current-voltage characteristic depending upon an environmental temperature, in a manner such that a higher temperature causes the element to have a lower emission threshold Vth, while a lower temperature causes it to have a higher emission threshold Vth. As a result, when driving at a constant voltage, an emission brightness thus obtained will undesirably change as L1, L2, and L3 depending on an environmental temperature. Consequently, as compared with an emission brightness set at a room temperature, a low temperature will cause a relative brightness to decrease and thus forms a dark state, while a high temperature will cause the relative brightness to increase and thus forms a bright state, hence rendering it impossible to maintain an acceptable displaying performance.
In order to solve the above problems, Japanese Unexamined Patent Application Publication Nos. 2001-223074 and 2002-304155 have suggested an improved technique for falsely realizing a constant-current driving by controlling a driving voltage while still maintaining a constant-voltage driving.
The above-mentioned technique can be explained with reference to FIG. 1B. Namely, a self-emission display apparatus has a self-emission element section J1 which is driven by a data line (anode) driving circuit J2 and a scanning line (cathode) driving circuit J3. The self-emission element section J1 includes a monitor element section JIB in addition to a display element section J1A. An electric current (monitor current) flowing through the monitor element section JIB is detected by a current detecting circuit J4, while a voltage adjusting circuit J6 is adjusted in accordance with an output from a correcting circuit J5 in a manner such that the monitor current will become equal to a predetermined current, thereby controlling a driving voltage applied to the data line driving circuit J2.
According to the prior art mentioned above, if a driving voltage of the display element section J1A is controlled in accordance with an operating state of the monitor element section JIB, it is possible to make constant a driving current of the display element section J1A which is driven by a constant voltage, without being affected by a temperature, thereby rendering a displaying performance not depending on an environmental temperature. Moreover, if a lighting rate of the monitor element section is adjusted so as to make a life characteristic of the monitor element section to be substantially equal to that of the display element section, it becomes possible to perform a control for increasing a driving voltage so as to avoid a brightness reduction to some extent, thereby making it possible to deal with a brightness reduction caused by an increased internal resistance (which is formed due to a long-term use).
However, a self-emission element such as an organic EL element has been known to have a degradation property generally shown in FIG. 2. That is, a brightness-current performance will become deteriorated with the development of the degradation shown in FIG. 2A, and a current-voltage performance will have the similar problem shown in FIG. 2B. On the other hand, it is possible to control a driving voltage of the display element section J1A according to an operating state of the monitor element section J1B so as to deal with a self-emission element showing the foregoing degradation as in the forgoing prior art, thereby effecting a control for increasing the driving voltage to deal with a decreased brightness. However, there is still a problem that the brightness will decrease with the passing of driving time due to a deterioration in the brightness-current performance.
Moreover, with regard to different materials exhibiting different emission colors, brightness reductions with the passing of driving time will have different velocities, as shown in FIG. 2C. For example, during a color displaying based on a mixed coloring involving a plurality of different colors, if driving voltages for effecting light emissions of various different colors are corrected in only one manner, a color balance set in advance will get collapsed due to different brightness reduction velocities with the passing of driving time, thus rendering it impossible to provide a desired color displaying. In particular, when a white color is displayed based on a mixed coloring involving R (red), G (green), and B (blue), there will be a distortion in color taste, resulting in a deteriorated picture displaying.